This invention relates generally to motors. In particular, it relates to a drive circuit for a three-phase brushless motor.
A conventional three-phase brushless motor drive circuit generally employs a 120.degree. switching system. However, the system suffers from difficulties that both acoustic noise and electrical noise are generated. Furthermore, the resultant torque is not uniform because a coil switching operation is carried out. In order to eliminate the difficulties, an analog energization system as shown in FIG. 1 has been proposed. Hall elements 1, 2 and 3 receive current through resistors 6 and 7 from power sources 4 and 5. They are positioned so that they detect magnetic flux from a rotor magnet on the motor. The output voltages of the Hall elements 1, 2 and 3 are amplified by amplifiers 9, 10 and 11 and are then applied to drive coils 12, 13 and 14, respectively. The current through the coils 12-14 cause the rotor magnet 8 to rotate. If it is assumed that, as shown in FIG. 2, the output voltages u, v and w of the Hall elements 1, 2 and 3 vary sinusoidally with the electrical angle .theta. of the rotor 8, then the voltage can be represented by u=sin .theta., v=sin (.theta. -2/3.pi.) and w=sin (.theta.-4/3.pi.). The normalized output waveforms of the amplifiers 9, 10 and 11 are then represented by u=sin .theta., v=sin (.theta.-2/3.pi.) and w=sin (.theta.-4/3.pi.), respectively. The torque conversion functions K.sub.u, K.sub.v and K.sub.w of the different phases are represented by K.sub.u =sin .theta., K.sub.v =sin (.theta.-2/3.pi.) and K.sub.w =sin (.theta.-4/3.pi.). As a result the torque of T.sub.m of the motor is: ##EQU1## That is, an ideal motor would produce no torque ripple, as shown in FIG. 2.
However, in the analog energization system, in general the outputs of the Hall elements fluctuate in amplitude, phase and offset among three phases thereof. Therefore, in practice, the outputs, being affected by these factors, produce torque ripple. A more realistic expression for the output of a representative amplifier 10 is V=K sin (.theta.-2/3.pi.+P)+L, (where K is the amplitude shift, P is the phase shift, and L is the offset). The voltage waveforms and the resultant ripples are illustrated in FIGS. 3A-3D for the realistic voltage waveform V but with the voltages U and W assumed to be ideally matched. FIG. 3A shows the ideal state (K=1, P=0, and L=0) for which no torque ripple is produced. FIG. 3B shows the case where the peak value of the phase V is 70% of that for U and W (K=0.7). The resultant torque ripple is 22.2%. FIG. 3C shows the case where the offset L of the phase V is 20%. The resultant torque ripple is 26.7%. FIG. 3D shows the case where the phase shift P is 15.degree.. The resultant torque ripple is 17.6%. In the cases shown in FIGS. 3B and 3C, there is a difference r between the peak values of the phase V and the peak values of the phases U and W and therefore the operating voltage range is reduced when it is clipped to the supply voltage. Furthermore, the three-phase balancing condition U+V+W=0 cannot be maintained. To counteract this problem, the power sources 4 and 5 are replaced by a single power source. However with a single power source, the imaginary ground potential is not well defined so that the operation of the system becomes unstable.